Soil–Water–Vine Relationships and Water Management - Soils for Fine Wines - page 150

Box 6.6 Infiltration Rate

The infiltration rate IR is expressed as a volume of water/area of soil
surface/time (e.g., mm/hr). When the soil's potential IR is greater than the rainfall
intensity, all the rain should infiltrate and the actual IR is then set by the rainfall
intensity. The cumulative infiltration I under this condition is given by
I (mm)

t (B6.6.1)
However, the soil's potential IR usually changes during a rainfall event (see
fig. 6.4). If rain continues to fall at a steady rate, the soil's actual IR may become
less than this rainfall rate, and surface runoff may occur. As with rainfall, the
runoff rate is expressed as a volume/area of surface/time in units of mm/hr.
Subsurface lateral flow, if it occurs, is calculated the same way.
Suppose that the surface runoff rate is R s (mm/hr) from an area A (ha) of a
vineyard block. The total volume V s (m 3 ) of water discharged in time t (hr) from
the block is given by
R s

IR

A t 10,000

V s (B6.6.2)

1,000
If the area of the block is 1 ha and the runoff period 1 hr, the volume
discharged is simply 10 R s m 3 . The conversion to American units of acre-feet is
given in appendix 15.

When the infiltration rate falls below the rainfall intensity, ponding occurs
on the soil surface. Initially, this ponding may be confined to small depressions,
which creates favorable conditions for preferential flow down macropores (section
6.3.3). But once the surface detention reaches a critical value, surface runoff oc-
curs (box 6.6). In soils that have a duplex profile, downward water movement
may be impeded at the top of a relatively impermeable B horizon. A zone of tran-
sient saturation can then develop (called a perched watertable ), and water begins
to flow laterally. This is referred to as subsurface lateral flow (box 6.5). Clearly, the
speed of both surface and subsurface flow depends on the resistance offered to the
flowing water and on the slope of the land, which determines the head gradient.

Water Redistribution in the Soil Profile
The velocity of water flow through a cylindrical pore varies with the square of the
pore radius (i.e., r 2 ). Hence, we would expect water to move 100 times faster
through water-filled pores of 1-mm radius than through pores of 0.1-mm radius
(other factors being the same). However, factors such as the tortuosity of pore
pathways and their surface roughness can vary and influence the rate of flow. Also,
a soil has a range of pore sizes, so that as the soil dries, the larger, more conduc-
tive pores drain of water, and the average radius of pores still conducting water
decreases. Water columns in the soil become discontinuous, so the pathways for
water movement become more tortuous. Overall, the rate of water flow through
the soil decreases markedly as the water content decreases.
These effects are expressed quantitatively by Darcy's Law, which relates the
rate of water flow to the driving force for water movement through a propor-
tionality coefficient called the hydraulic conductivity K (box 6.7).

6.3.2